considered Zoologically and Geologically. 219 



delight in swimming. The naked mollusca of the genus 

 Eolida swim with considerable rapidity, and are continually 

 moving about when in their native element. Those who judge 

 of these animals from seeing them on or under a stone at low 

 water can form but a slight idea either of their beauty or ha- 

 bits. There is another instance of the variable appearance of 

 a mollusc which I would mention. The spines of Spatangus 

 purpureus (abundant on the bank) are some years covered 

 with the parasitic Montacuta substriata, and at other seasons 

 quite clear. 



The following species are always found on the bank, and 

 occasionally in considerable numbers : 



Fusus antiquus. Venus cassina. 



corneus. fasciata. 



Bamfius. — ^ virginea. 



Nucula margaritacea. Kellia suborbicularis. 



Pectunculus pilosus. Velutina laevigata. 



The Fusus antiquus is the small form of that shell called 

 Fusus norvegicus by some authors. It seems confined to cer- 

 tain spots on the bank. Most univalve shells of the siphono- 

 stomatous tribes are gregarious and plentiful in confined lo- 

 calities. Most of the above-mentioned species are gregarious, 

 both univalve and bivalve. It is remarkable that young spe- 

 cimens of Venus cassina and fasciata rarely occur, and when 

 found are almost always dead shells, whilst of Venus vi7'ginea 

 the only living specimens taken are young ones. In the lat- 

 ter case it may be accounted for by supposing that Venus vir- 

 ginea, being a borer in mud, buries itself deeper when full- 

 grown than when young. Kellia suborbicularis lives in little 

 families of all ages in the mud-filled cavities of dead Verms 

 virginea. Along with it is occasionally associated Amphidesma 

 tenue. 



Always found on the bank, though never in great numbers, 

 are the following : 



Ostrsea edulis. Murex erinaceus. 



' Pecten maximus. Capulus hungaricus. 



Lima fragilis. Natica Alderi. 



Pecten obsoletus. Venus exoleta. 



Cardium laevigatum. ovata, 



Psammobia tellinella. Mya truncata. 



r2 



